The multiple antennae input multiple antennae output (abbreviated as MIMO) technology is a key technology of the 3rd Generation (abbreviated as 3G), 4th Generation (abbreviated as 4G), and even the future broadband wireless communication. The MIMO technology can be divided into two categories: open loop MIMO technology and closed loop MIMO technology. The closed loop MIMO technology can significantly improve the system capacity, but it needs to obtain the information of the transmission channels. In the closed loop MIMO technology, the transmitters select suitable transmission modes according to the characteristics of the transmission channels.
In the LTE protocol, when the user equipment (abbreviated as UE) works in closed loop MIMO mode with space multiplexing, the UE needs to select an optimal (for example, which maximizes the system throughput) pre-coding matrix from a set of pre-coding matrices specified in advance and use this pre-coding matrix to transmit signals. When the number of transmission antennas is greater than that of the layers of the transmission signal, the selection of the pre-coding matrix has an optimal solution which can be proved theoretically, such as maximized system capacity selection method and the selection method of maximizing the correlation with the right eigen-matrix of the channel. However, these methods are inapplicable when the number of transmission antennas is equal to that of the layers of the transmission signal. In the LTE system, one commonly used antenna configuration is that a base station (NodeB) is equipped with 2 transmission antennas and the UE with 2 receiving antennas. The NodeB uses the spatial multiplexing mode and transmits 2 layers of signal at the same time. The UE needs to select 1 optimal pre-coding matrix from 2 pre-coding matrices for feeding back to the NodeB. Under this situation, the number of transmission antennas is equal to that of the signal layers. The LTE proposition provides a method for selecting an optimal pre-coding matrix when the number of transmission antennas is equal to that of the signal layers, which is as follows:i=arg max(log(1/c(i)00)+log(1/c(i)11))
This method is based on the maximized system capacity criterion for an MMSE receiver, wherein c(i)00 and c(i)11 are respectively the mean squared error (abbreviated as MSE) of the 0th and 1st layer of the signal when using pre-coding matrix i. This method effectively maximizes the MSE difference of the 2 layers. The performance of this algorithm is relatively poor when being applied in an LTE system when the number of transmission antennas is equal to that of the signal layers. Although it is possible for a layer of signal with low MSE to pass through when the spatial characteristics of the channel are poor, it would cause another layer with high MSE to be received in error when the spatial characteristics of the channels are good. In addition, there is another method for selecting a pre-coding matrix applied in an LTE system when the number of transmission antennas is equal to that of the signal layers, in which the MSE difference of the 2 layers is minimized.
However, in practical application, it is often difficult to decide when to use the maximized MSE difference method and when to use the even MSE method. That is, it is difficult to select the better pre-coding matrix. Therefore, the system performance is compromised.